This may be due to a known issue between Cisco IP phone (7910, 7940 and 7960) and
some Cisco switches.
The Cisco IP phone is designed with certain loop-back capability when power is not given so it can be detected by Cisco in-line-power switch (ex. 3550-power)and power can be applied. Because this loop-back capability, "sometimes" it can fool the Ethernet PHY of the switch and PHY will report link is up, then it can be detected by switch's error-disable feature that port is in the "loop-back state" and been shut down.
This loop-back capability issue can be found on some 7910, 7940 and 7960 IP phones when
connect to several type of Switches (3550 is one).
Ensure the IP Phone's external power supply is not removed while connected to the switch.
Most of the time the workaround above resolve the issue. Also you can try with this fix:
- switch(config)#errdisable recovery cause loopback
- switch(config)#errdisable recovery interval 30
- switch(config)#no errdisable detect cause loopback
Make sure Ethernet cable is removed from switch before AC power is
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